1. In an effort to understand the molecular mechanisms involved in the transformation process mediated by myelocytomatosis virus (MC29), molecular cloning of the complete proviral genome was undertaken. We have successfully cloned the proviral genome in lambda Charon 21A phage. Studies are now being performed to test the transforming activity of the viral genome using various cell lines derived from mouse, rat, and avian species. In vitro mutagenesis of the MC29 proviral genome is currently being carried out in an effort to understand the molecular mechanisms involved in the transformation process mediated by this genome. A recombinant virus containing the v-myc oncogene with Abelson long terminal repeats (LTRs) was constructed and live virus rescued upon cotransfection. Studies of the in vitro and in vivo effects of this virus in mouse cell lines are currently being carried out. 2. In an effort to delineate the molecular mechanisms involved in the rearrangement of c-myb locus in mouse plasmacytoid lymphosarcomas induced by Abelson murine leukemia virus, molecular cloning of mouse c-myb locus was undertaken and the comparison of it with the molecular clone of its counterpart in ABPL-2 tumors reveals that the rearrangement observed in the c-myb locus of the tumor line is due to insertion of a defective Moloney murine leukemia proviral genome upstream to v-myb-related sequences.